The disclosure of Japanese Patent Application No. 11-368007, filed on Dec. 24, 1999 including, the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to an evaporative fuel control apparatus and a control method therefor for an internal combustion engine for purging evaporated fuel in a fuel tank into an intake passage of the internal combustion engine.
2. Description of Related Art
There is a conventional evaporative fuel control apparatus for collecting evaporated fuel (vapor) generated in a fuel tank in a canister and purging the collected vapor into an intake passage of an internal combustion engine from the canister as required.
In the conventional evaporative fuel control apparatus, in order to suppress an adverse effect on an air-fuel ratio of the internal combustion engine, the purge passage is provided with a purge-amount adjusting valve, such that the purge amount is controlled in accordance with an operation state of the engine by opening and closing the purge-amount adjusting valve.
In general, purge air that is purged into the intake passage from the canister includes not only purge air that has been once adsorbed by adsorbent of the canister and then separated from the adsorbent (separated vapor, hereinafter), but also purge air where the fuel component has not been sufficiently adsorbed by the adsorbent and is purged into the intake passage from the fuel tank through the canister (tank vapor, hereinafter). Therefore, when the purge is controlled while taking only the separated vapor into consideration, this purge may disturb the air-fuel ratio due to the tank vapor or may adversely affect the emission due to disturbance of the air-fuel ratio.
To avoid the above inconvenience, there is another conventional apparatus such as that disclosed in Japanese Patent Application Laid-open No. 9-303219 which has an evaporative fuel control apparatus for controlling the purge while taking both the separated vapor and tank vapor into consideration.
According to the other conventional apparatus, a purge ratio based on the separated vapor amount and a purge ratio based on the tank vapor amount are calculated as upper limit values of the purge ratio (purge amount/intake air amount), and the smallest of the upper limit values of the purge ratio is set as a maximum purge ratio so as to limit the upper limit of the purge amount.
Further, according to the other conventional evaporative fuel control apparatus, a tank internal pressure control valve which is opened when the pressure in the fuel tank exceeds a predetermined pressure is provided midway in a passage (vapor passage) for bringing the fuel tank and the canister into communication with each other. By providing such a valve, it is possible to prevent the vapor from being excessively introduced into the canister from the fuel tank, such that a predetermined adsorbing ability in the canister can be maintained.
However, if the tank internal pressure control valve is opened during execution of the purging operation and a large amount of vapor is introduced into the canister from the fuel tank through the vapor passage, problems such as those below which cannot be ignored arise.
That is, if a large amount of vapor is introduced into the canister from the fuel tank, the amount of the introduced vapor will exceed the adsorbing limit and this vapor exceeding the adsorbing limit will be passed into the intake passage without adsorption or purge added. This may then disturb the air-fuel ratio or may adversely affect the emission due to a disturbance of the air-fuel ratio.
Thus, in all of the conventional apparatus, the vapor amount is limited based on the tank vapor, and an excessive increase in tank vapor, that may be caused by opening the tank internal pressure control valve, is not taken into consideration, and it is not possible to cope with a deterioration in the air-fuel ratio control caused by the disturbance of the air-fuel ratio.
It is one object of the invention to provide an evaporative fuel control apparatus for an internal combustion engine capable of suppressing deterioration in the air-fuel ratio control which may be caused by vapor in the fuel tank being introduced into the canister during execution of the purging operation.
According to a first aspect of the invention, there is provided an evaporative fuel control apparatus for an internal combustion engine having a canister for collecting evaporated fuel generated in a fuel tank, an introducing valve in a communication passage between the canister and the fuel tank, which introduces the evaporated fuel in the fuel tank into the canister, a purge amount adjusting valve in a purge passage between the canister and an intake passage of the internal combustion engine, which adjusts an amount of purge to be purged from the canister to the intake passage and a controller that controls the purge amount adjusting valve based on an operating state of the engine. The controller decreases the opening amount of the purge amount adjusting valve to less than a target opening amount, based on the operating state of the engine, when the introducing valve is opened.
With the above arrangement, when the introducing valve is opened, the opening amount of the purge amount adjusting valve is decreased so that the purge amount is controlled to be smaller than an amount corresponding to the target purge ratio. Therefore, even if vapor in the fuel tank is introduced to the canister as the introducing valve is opened such that the vapor amount and concentration of the vapor in the canister are temporarily increased, it is possible to suppress a deterioration in air-fuel ratio control.
In the above aspect, it is possible that when the controller decreases the opening amount of the purge amount adjusting valve to less than the target opening amount, the controller fully closes the purge amount adjusting valve.
With this arrangement, it is possible to more reliably suppress the deterioration in the air-fuel ratio control.
In the above aspect, it is possible that the controller decreases the opening amount of the purge amount adjusting valve to less than the target opening and then gradually increases the opening amount up to the target opening amount.
With this arrangement, the opening amount of the purge amount adjusting valve is decreased to less than the target opening amount and then the opening amount is gradually increased up to the target opening amount based on the operating state of the engine. Therefore, it is possible to suppress abrupt variation in vapor amount and concentration of vapor to be purged by the intake passage, to suppress the deterioration in the air-fuel ratio control, and to ensure a predetermined purge amount corresponding to the target purge amount.
In the above aspect, the apparatus may further include a pressure sensor for detecting an internal pressure of the fuel tank. The controller may judge a valve-opening timing of the introducing valve based on the internal pressure of the fuel tank detected by this pressure sensor, and decrease the opening amount of the purge amount adjusting valve to less than the target opening amount based on the judged valve-opening timing.
With this arrangement, even if the introducing valve is a valve which cannot directly detect the opening or closing state thereof such as a differential pressure regulating valve which is opened and closed in accordance with a differential pressure between the internal pressure of the fuel tank and a predetermined reference pressure, it is possible to easily and reliably judge the valve-opening timing of the introducing valve, to precisely grasp the timing for introducing the vapor into the canister from the fuel tank, and appropriately suppress deterioration in the air-fuel ratio control.